So-called open-fire heaters are, as is known, very attractive to the users, this attraction being the result of the visual effect of the flames during combustion.
Unfortunately, it is also known that heaters of this type exhibit very mediocre efficiency and attempts at creating fireplace fires or stoves which have a window or the like have therefore been made. Such a window consists of a frame containing one or more transparent or possibly translucent panels, generally made of glass or vitreoceramic, or even sometimes opaque panels if a visual effect is not sought, these types of panels having a coefficient of expansion which, in general, differs from that of the frame.
When the panel is made of a single piece, sealing between this panel and the frame is obtained by a somewhat flexible gasket acting under pressure, tension or sliding.
Now, what happens is that the temperature variations cause differential expansion between the frame and its contents, the coefficient of expansion of which differs from that of the frame. In theory, these expansions should be left to take effect with a certain amount of freedom in order to avoid excessive mechanical loadings which could cause cracking or breakage.
This is as true of flat panels as it is of bowed, curved or prismatic panels, provided they are still of one piece.
When the transparent or translucent constituent part is made up of a number of contiguous pieces or panels, a gap always occurs between them because the expansion of the frame exceeds that of the material of which the panels are made. This gap is eminently prejudicial to sealing and causes other drawbacks, especially emanations of gases, deposits and soiling.
In document EP 0 446 792 A2, it has been proposed that there be used, for a tube intended for a color television which has an evacuated envelope and a fixture comprising a mask. [sic] The device for fitting the mask is produced, for a better ability to withstand shocks and vibrations, in the form of a system made of bimetallic materials.
A similar system is described in document U.S. Pat. No. 3,935,497-A.
It should first of all be noted that such a device comes from a technical field that is entirely different from the heaters that are the subject of this application. In any case, tubes of this type do not involve differences in thermal expansion which are of the same order of magnitude as those of the invention, and the problems of sealing between the frame and one-piece panels and especially panels made up of several pieces that it contains, do not arise.